1. Field of the Disclosure
The present disclosure relates to electronic devices including discontinuous storage elements within a dielectric layer and, more particularly, to nonvolatile memory cells that include discontinuous storage elements within a dielectric layer and processes of forming them.
2. Description of the Related Art
An electronic device can include a nonvolatile memory cell including discontinuous storage elements (“DSEs”) within a dielectric layer. Such DSEs can be silicon nanocrystals. Silicon nanocrystals are desirable because a defect in the dielectric layer can compromise the ability of one silicon nanocrystal to store charge, without affecting the ability of another silicon nanocrystal to store charge. Thus, the nonvolatile memory cell including silicon nanocrystals can continue to function with a defect in the dielectric layer when a floating gate electrode with such a defect would fail to store charge.
However, silicon nanocrystals can have limitations. For example, quantum effects in silicon nanocrystals below 2 nm in size can negatively affect the ability of the silicon nanocrystals to store charge. Also, silicon nanocrystals can be so large that they merge into a continuous mass and have the same vulnerability to be compromised by a dielectric defect as a floating gate electrode. Another limitation of silicon nanocrystals is that the dielectric material used to encapsulate the silicon nanocrystals can be conventionally deposited and of lower quality than a thermally grown dielectric material. This lower-quality deposited dielectric material can be degrade more quickly due to charge trapping in the dielectric material, trap assisted charge loss from the silicon nanocrystals through the deposited dielectric material, or any combination thereof than thermally grown dielectric material.
Silicon nanocrystals can be formed using a low energy (at most 2 KeV) ion implantation of silicon into an oxide layer. Ion implantation can be followed by an anneal process to allow the implanted silicon to coalesce into silicon nanocrystals. The energy of the implant controls the projected range at which the silicon is placed within the oxide layer and the implant dose can determine the final thickness and structure of the resulting charge-storage layer. However, at 1 KeV energy, a peak silicon dose is predicted to lie at a depth of 4 to 5 nm, with a measurable amount at the surface of the oxide layer. Thus, the thickness of the oxide layer receiving the ion implant is at least in part determined by the ability to control the depth of the implant at low energy levels. Also, below a limit of 5E15 atoms per cm2, the implanted silicon is predicted to coalesce into spheres of up to 3 nm in diameter. However, at a dose of 1E16 atom per cm2, implanted silicon is predicted to form a spatially connected layer of silicon within the oxide layer. In one example, ion implanting using an energy of 2 KeV and a dose of 5E16 atoms per cm2 forms a continuous silicon mass of approximately 4.5 nm in thickness.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention. The use of the same reference symbols in different drawings indicates similar or identical items.